In designing a valve cover, it is important to provide the valve cover with sufficient strength and adequate sealing capabilities while, at the same time, addressing the problem of damping noise-producing vibrations and the problem of producing the cover at as low a cost as possible. In this regard, while there are many adequate valve covers in existence, they all have one or more shortcomings in relation to the above indicated factors of strength, sealing, vibration damping and cost minimization.
U.S. Pat. No. 4,164,927, to Congram et al discloses a reinforced stamped metal valve cover in the form of an elongated trough-like configuration that is stamped from a single sheet of metal. For strength purposes, generally U-shaped strengthening members are attached to the underside of the housing. The legs of the U-shaped strengthening members extend the full height of the trough-like housing and the legs are connected by a portion of the U-shape that extends across the bottom wall of the trough-like shape (underside of the top wall of the cover). These strengthening members are positioned at four locations, two near each longitudinal end of the housing, and the free ends of their legs coact with a flange-like leg of the housing side walls to form receiving pockets for a conventional ring-like gasket. The cover is secured to the rocker housing by nuts that are threaded onto threaded studs which extend upwardly from the rocker arm mounts through holes in the top wall of the valve cover.
However, such a valve cover has a number of drawbacks. Firstly, the cost of manufacturing such a stamped valve cover is increased by virtue of the fact that the four strengthening members must be separately welded onto the housing part. Furthermore, since loading of a flat gasket will produce spreading in a direction perpendicular to the direction in which the load is applied, the fact that the gasket of the Congram et al valve cover is restrained against inward spreading at only four points will enable non-uniform spreading of the gasket when the valve cover is secured in place. This problem is compounded by the lack of any strengthening members in the longitudinally central area of the housing which enables the longitudinally extending side walls to spread apart under action of the securing force pressing down on the top of the trough-like housing. As a result, it cannot be insured that an adequate sealing pressure will exist around the entire periphery of the cover and thus oil leakage across the gasket seal can become a problem.
Additionally, since there is a direct metal-to-metal interface between the engine and stamped metal valve cover in the Congram et al arrangement, a conventional metal ring washer being the only interface between the top of the valve cover and the fastening nut threaded onto the rocker mounted stud, noise-producing vibrations will be a problem even though an elastomeric gasket interface exists between the peripheral bottom edge of the valve cover and the top surface of the head upon which it is mounted.
In Timour U.S. Pat. No. 4,027,644, assigned to the assignee of the present invention, an isolated engine cover is disclosed wherein the problem of damping noise-producing vibrations is specifically addressed. Timour utilizes a continuous, vertically elongated, elastomeric seal element which extends from a groove formed around the periphery of the engine cover in conjunction with mounting bolts having resilient fastener assemblies in order to prevent any direct or indirect transfer of vibrations between the engine and the cover. Although the arrangement of this patent provides an adequate means for diminishing the noise emanating from the cover for the rocker arms, it is subject to oil leakage. That is, the width of the vertically elongated elastomeric element which seals the periphery of the cover is not sufficient to prevent oil leakage thereacross during periods of heavy engine vibration.
In order to provide a more substantial sealing arrangement around an engine cover for a rocker housing, in Kasting et al, U.S. Pat. No. 4,471,731, also assigned to the assignee of the present invention, the vibration damping concept of Timour is further developed in conjunction with a plastic rocker arm cover in order to achieve an effective sealing arrangement as well as effective noise damping capacity. In particular, the plastic rocker cover is secured about its periphery by a plurality of vibration-damping bolt assemblies with oil leakage being minimized by a flat ring-like gasket being inserted between the rocker housing and the rocker cover and with a drip lip extending around the inner periphery of the cover so as to bite into the gasket when the bolt assemblies are tightened in order to increase the sealing capacity of the gasket. Additionally, for strengthening purposes, the cover of the Kasting et al patent has longitudinal and lateral reinforcing ribs on its undersurface.
While the Kasting et al rocker cover provides an effective leak-proof, noise-damped cover arrangement, it is relatively expensive. That is, because the cover is peripherally secured by bolts, a relatively large number of bolts (14 in the illustrated embodiment) is required and its vibration damping structure requires that each securing bolt be surrounded by a steel sleeve which is in turn surrounded by a custom-molded elastomeric grommet that is inserted into the bolt holes in the peripheral flange of the cover.
In addition to the above-noted facts, it is pointed out that continuous annular gasket seals require proper positioning and precision manufacture and if they are stamped from a sheet they produce excessive waste while the cost of production is relatively high if molded. Furthermore, continuous, annular gasket seals are normally formed of a silicon rubber and are not reusable because of the fact that, upon exposure to oil, the rubber material swells so that, if the valve cover is removed, the seal expands longitudinally and will no longer fit. Thus, the fact that the gasket seal must be replaced any time the valve cover is removed is another disadvantage of known valve cover arrangements.